The present invention relates to the use of a ribonuclease of the T2 family or a polynucleotide encoding same for preventing, inhibiting and/or reversing proliferation, colonization, differentiation and/or development of abnormally proliferating cells in a subject. The present invention further relates to pharmaceutical compositions containing, as an active ingredient, a ribonuclease of the T2 family or a polynucleotide encoding same for treating proliferative diseases or disorders in general and cancer in particular.
There is an ongoing interest, both within the medical community and among the general population, in the development of novel therapeutic agents for the treatment of cell proliferative diseases and disorders such as cancer.
Agents that display anti-proliferative, anti-colonization, anti-differentiation and/or anti-development properties against mammalian cells can potentially be used as anti-cancer drugs. As such, these agents are widely sought for from both natural as well as synthetic sources.
RIBASES are ribonucleases (RNases) which display a biological activity which is distinct from their ability to degrade RNA. RIBASES and their structural homologous are known to effect a large number of cellular reactions (Rybak, M. et al., 1991, J. Biol. Chem. 266:21202-21207; Schein, C. H. 1997 Nature Biotechnol. 15:529-536). EDN and ECP, two major proteins found in the secretory granules of cytotoxic eosinophyles (members of RNase A family) are thought to participate in the immune response. In self-incompatible plants stylar S-RNases (members of RNase T2 family), arrest pollen tube growth and thus prevent fertilization. RC-RNase, produced from Bullfrog oocytes, inhibits, in vitro, the growth of tumor cells such as the P388, and L1210 leukemia cell lines and is effective for in vivo killing of sarcoma 180, Erlich, and Mep II ascites cells (Chang, C-F. et al 1988, J. Mol. Biol 283:231-244). Some RNases display limited ribonuclease activity, an example of which includes angiogenins that stimulate blood vessels formation (Fett, J. W. 1985, Biochemistry 24:5480-5486).
Living organisms use extracellular RNases for defense against pathogens and tumor cells. For example, ECP is secreted in response to parasite attack (Newton, D L. 1992, J. Biol. Chem. 267:19572-19578) and displays antibacterial and antiviral activity. This activity is also displayed by Zinc-α2-glycoprotein (Znα2gp), an RNase present in most human body fluids including blood, seminal plasma, breast milk, synovial fluid, saliva, urine and sweat (Lei G, et al., 1998, Arch Biochem Biophys. Jul. 15; 355(2): 160-4).
The specific mechanism by which extracellular RNases function in cellular reactions is unknown.
The main barrier to the cytotoxic activity of some RNase is the cell membrane. ECP was found to form channels in both artificial and cellular membranes. Presumably, ECP released from the granule membrane along with EDN (eosinophylic RNase, which is responsible for cerebellar Purkinjie cell destruction) transfers EDN into the intercellular space. The entrance of the fungal toxin α-sarcin (a member of the RNase A family) into target cells depends upon viral infection which permeabilizes the cellular membrane (Rybak, M. et al., 1991, J. Biol. Chem. 266:21202-21207). It is also possible that RNases enter the cell via endocytosis. When the Golgi-disrupting drugs retinoic acid or monensin were used to artificially deliver BS-RNase into the cells, cytotoxicity increased dramatically (Wu Y, et al., 1995, J Biol. Chem. 21; 270(29):17476-81).
Cytotoxicity of RNases can be used for therapeutic purposes. Human RNase L is activated by interferon and inhibits viral growth. Expression of the gene for human RNase L together with that for a 2′5′-A synthetase in tobacco plants is sufficient to protect plants from cucumber mosaic virus and to prevent replication of potato virus Y. Human immunodeficiency virus-1 (HIV-1) induces blockade in the RNase L antiviral pathways (Schein, C. H. 1997 Nature Biotechnol. 15:529-536.). RNases can be fused with specific membranal protein antibodies to create immunotoxins. For example, fusion of RNase A with antibodies to the transferrin receptor or to the T cell antigen CD5 lead to inhibition of protein synthesis in tumor cells carrying a specific receptor for each of the above toxins (Rybak, M. et al., 1991, J. Biol. Chem. 266:21202-21207; Newton D L, et al., 1998, Biochemistry 14; 37(15):5173-83). Since RNases are less toxic to animals, they may have fewer undesirable side effect than the currently used immunotoxins.
The cytotoxicity of cytotoxic ribonucleases appears to be inversely related to the strength of the interaction between a ribonuclease inhibitor (RI) and the RNase. Ribonuclease inhibitor (RI) is a naturally occurring molecule found within vertebrate cells which serves to protect these cells from the potentially lethal effects of ribonucleases. The ribonuclease inhibitor is a 50 kDa cytosolic protein that binds to RNases with varying affinity. For example, RI binds to members of the bovine pancreatic ribonuclease A (RNase A) superfamily of ribonucleases with inhibition constants that span ten orders of magnitude, with Ki's ranging from 10−6 to 10−16 M.
A-RNases
ONCONASE, like RNase A and BS-RNase, is a member of the RNase A superfamily. Members of the RNase A superfamily share about 30% identity in amino acid sequences. The majority of non-conserved residues are located in surface loops, and appear to play a significant role in the dedicated biological activity of each RNase. ONCONASE was isolated from Northern Leopard frog (Rana pipiens) oocytes and early embryos. It has anti-tumor effect on a variety of solid tumors, both in situ and in vivo (Mikulski S. M., et al., 1990 J. Natl. Cancer 17; 82(2):151-3). ONCONASE has also been found to specifically inhibit HIV-1 replication in infected H9 leukemia cells at non-cytotoxic concentration (Youle R. J., et al., 1994, Proc. Natl. Acad. Sci. 21; 91(13):6012-6).
Although the RNase activity of ONCONASE is relatively low, it is accepted that the enzymatic and cytotoxic activities thereof are associated to some degree. It is believed that the tertiary structure of A-RNases differentiate between cytotoxic and non-cytotoxic types. For example, differences between the tertiary structure of ONCONASE and RNase A are believed to be responsible for the increased cytotoxicity observed for ONCONASE. ONCONASE, unlike RNase A, contains a blocked N-terminal Glu1 residue (pyroglutamate) which is essential for both enzymatic and cytotoxic activities. This unique structure enables ONCONASE to permeate into target cells (Boix E., et al., 1996, J. Mol. Biol. 19:257(5):992-1007). In addition, in ONCONASE the Lys9 residue replaces the Gln11 residue of RNase A, which is believed to effect the structure of the active site. Furthermore, differences in the amino acid sequence of the primary structure between ONCONASE and RNase A cause topological changes at the periphery of the active site which effect the specificity thereof (Mosimann S. C., et al., 1992, Proteins 14(3):392-400).
The differences in toxicity between A-RNases are also attributed to their ability to bind RI. Bovine seminal ribonuclease (BS-RNase) is 80% identical in its amino acid sequence to RNase A, but unlike other members of the RNase A superfamily, BS-RNase exists in a dimeric form. It has been shown that the quaternary structure of BS-RNase prevents binding by RI, thereby allowing the enzyme to retain its ribonucleolytic activity in the presence of RI (Kim et al., 1995, J. Biol. Chem. 270 No. 52:31097-31102). ONCONASE, which shares a high degree of homology with RNase A, is resistant to binding by RI. The RI-ONCONASE complex has a Kd at least one hundred million times less than that of the RI-RNase A complex. The lower binding affinity of ONCONASE for RI prevents effective inhibition of the ribonucleolytic activity and could explain why ONCONASE is cytotoxic at low concentrations while RNase A is not.
It is suggested that binding to cell surface receptor is the first step in ONCONASE cytotoxicity. Nothing is known about the nature of ONCONASE receptors on mammalian cell surfaces. ONCONASE may bind to cell surface carbohydrates as in the case of ricin, or it may bind to receptors originally developed for physiologically imported molecules like polypeptide hormones (Wu Y, et al., 1993, J. Biol. Chem. 15; 268(14):10686-93). In mice, ONCONASE was eliminated from the kidneys in a rate 50-100-fold slower than did RNase A. The slower elimination rate of ONCONASE is explained as a result of its higher ability to bind to the tubular cells and/or by its resistance to proteolytic degradation. The strong retention of ONCONASE in the kidneys might have clinical implications (Vasandani V. M., et al., 1996, Cancer Res. 15; 56(18):4180-6). ONCONASE may also bind to Purkinjie cells EDN receptors (Mosimann S. C., et al., 1996, J. Mol. Biol. 26; 260(4):540-52). The specificity of ONCONASE is also expressed in its tRNA preference. In rabbit reticulocyte lysate and in Xenopus oocytes it was discovered that ONCONASE inhibits protein synthesis via tRNA, rather than via rRNA or mRNA degradation. In contrast, RNase A degrades mostly rRNA and mRNA (Lin J J., et al., 1994, Biochem. Biophys. Res. Commun. 14; 204(1):156-62).
Treatment of susceptible tissue cultures with ONCONASE results in the accumulation of cells arrested in G1 phase of the cell cycle, having very low level of RNA contents (Mosimann S. C., et al., 1992, Proteins 14(3):392-400). In glioma cells ONCONASE inhibited protein synthesis without a significant reduction in cell density, showing that ONCONASE is also cytotoxic to cells in addition to being cytostatic (Wu Y., et al., 1993, J. Biol. Chem. 15; 268(14):10686-93). ONCONASE, combined with chemotherapeutic agents, can overcome multidrug resistance. Treatment with vincristine and ONCONASE increased the mean survival time (MST) of mice carrying vincristine resistant tumors to 66 days, compared to 44 days in mice treated with vincristine alone (Schein, C. H., 1997, Nature Biotechnol. 15:529-536).
Furthermore, some chemotherapeutic agents may act in synergy with ONCONASE. In tumor cell lines of human pancreatic adenocarcinoma and human lung carcinoma treated with a combination of ONCONASE and tamoxifen (anti-estrogen), trifluoroperazine (Stelazine, calmodulin inhibitor) or lovastatin (3-hydroxyl-3-methylglutatyl coenzyme A (HMG-CoA) reductase inhibitor) a stronger growth inhibition was observed than cells treated with ONCONASE alone (Mikulski S. M., et al., 1990, Cell Tissue Kinet. 23(3):237-46). Thus, a possibility of developing combination therapy regiments with greater efficiency and/or lower toxicity is clear.
Bovine seminal RNase is a unique member of RNase A family, since it is the only RNase containing a dimmer of RNase A-like subunits linked by two disulfide bridges. In addition, it maintains allosteric regulation by both substrate and reaction products. The regulation occurs at the cyclic nucleotide hydrolysis phase. It has the ability to cleave both single- and double-stranded RNA. BS-RNase is highly cytotoxic. It displays anti-tumor effect in vitro on mouse leukemic cells, HeLa and human embryo lung cells, mouse neuroblastoma cells, and human fibroblasts and mouse plasmacytoma cell lines. When administrated in vivo to rats bearing solid carcinomas (thyroid follicular carcinoma and its lung metastases), BS-RNase induced a drastic reduction in tumor weight, with no detectable toxic effects on the treated animals (Laccetti, P. et al., 1992, Cancer Research 52:4582-4586). Artificially monomerized BS-RNase has higher ribonuclease activity but lower cytotoxicity than native dimeric BS-RNase (D'Allessio G., et al., 1991, TIBS:104-106). This, again, indicates the importance of molecular structure for the biological activity. It seems that like ONCONASE, BS-RNase binds to recognition site(s) on the surface of the target cells, prior to penetration into target cells.
In addition to being cytotoxic, BS-RNase is also immunorepressive. BS-RNase can block the proliferation of activated T cells, and prolong the survival of skin grafts transplanted into allogenetic mice. The immunorepressive activity of SB-RNase is explained by the need to protect sperm cells from the female immune system.
T2-RNases
In plants, self-compatibility is abundant and is effective in preventing self-fertilization. Pollen carrying a particular allele at the S locus, which controls self-incompatibility, is unable to fertilize plants carrying the same S-allele. In many self-incompatible plants, especially members of Solanaceae and Rosaceae, S-RNase, a member of the T2-RNase family is secreted by the female organs. S-RNase specifically recognize self-pollen and arrest its growth in the stigma or style before fertilization occurs (Clarke, A. E. and Newbigin, E., 1993, Ann. Rev. Genet. 27:257-279) it is believed that the arrest of pollen tube growth is a direct consequence of RNA degradation, however the mode of S-RNase entrance into the tube cell is still obscure.
Members of RNase T2 family were first identified in fungi (Egami, F. and Nakamura, K. 1969, Microbial ribonucleases. Springer-Verlag, Berlin). Since, they were found in a wide variety of organisms, ranging from viruses to mammals. In particular, T2-RNases show much broader distribution than the extensively described RNase A family. However, the in vivo role of T2-RNases in mammalian cells is still not known.
In microorganisms, extracellular T2-RNases are generally accepted to contribute to the digestion of polyribonucleotides present in the growth medium, thereby giving rise to diffusible nutrients. They may also serve as defense agents (Egami, F. and Nakamura, K., 1969, Microbial ribonucleases. Springer-Verlag, Berlin).
In plants, T2-RNases play a role in the pollination process, by selectively limiting the elongation of pollen tubes racing towards the ovules (Roiz, L. and Shoseyov, O., 1995, Int. J. Plant Sci. 156:37-41, Roiz L. et al., 1995, Physiol. Plant. 94:585-590). To date, the mechanism by which these RNases affect pollen tubes is unclear.
Thus, there exist few examples of cytotoxic ribonucleases which can be effectively used as cancer treatment agents. New ribonucleases with anti-proliferation, anti-colonization, anti-differentiation and/or anti-development activities toward mammalian cells are needed to enhance the spectrum of therapeutic agents available for treatment of human cancers, to thereby open new horizons in the field of cancer treatment.
Apoptosis and Disease
Cell death can occur through two different processes, termed necrosis and apoptosis, which can be distinguished by specific sets of functional and morphologic characteristics. Necrosis is a traumatic cell death that occurs as a response to injurious agents in the extracellular surroundings e.g. hypoxia, hyperthermia, viral invasion, exposure to toxins, or attack by pathogens. The ion and water pumps in the plasma membrane lose their abilities to maintain concentration gradients, the cells and mitochondria swell and eventually burst, leaking cellular constituents and leading to an inflammatory response in the surrounding tissue. In apoptosis, or programmed cell death (PCD), cells are induced to self-destruct via an intrinsic genomic program. The cells shrink and the mitochondria break down and release cytochrome c. The nuclear DNA is gradually degraded into monomers and multimers of about 200 bases. Eventually, the cells undergo blebbing, and break into small, membrane-wrapped fragments called apoptotic bodies which are engulfed by nearby phagocytotic cells (Rudin C M and Thompson C B. 1997. Annu Rev Med. 48:267-81; Chamond R. R. et al. 1999. Alergol Immunol Clin. 14:367-374). Significantly, no inflammatory response in surrounding tissues is elicited. The difference between death forms is summarized in the following Table 1.
TABLE 1Apoptosis vs. NecrosisNecrosisApoptosisEtiologyAcute cell injury due to extracellularVarious intracellular or extracellular stimulistimuliCharacterPathologicPhysiologic or pathologicDistributionGroups of cells or patchesWildly scattered isolated cellsof tissuesEnergy requirementPassive process (ATP-independent)Active process (ATP-dependent)Morphologic featuresSwelling of the cytoplasm.Shrinkage of the cytoplasm,Membrane lysis with loss of cell andExternalization of phosphatidylserine.organelles contents.Cell membrane blebbing to formapoptotic bodies encompassingcytoplasm and organelles.Chromatin condensation and DNAfragmentation into multimers of 200bases.Reaction of theInflammationPhagocytosis without inflammatory reactionsurrounding tissue(Nikitakis N.G. et al. 2004. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 97:476-90.)
Apoptosis plays a central role in the regulation of homeostasis, in normal, and in pathological processes. During embryogenesis apoptosis is responsible for the disappearance of the tadpoles tail, the differentiation of fingers and toes, and the removal of unnecessary neurons in the brain. It is also involved in disassembly of the endometrium at the menstruation, and in the aging process (Herndon F J. et al. 1997. Mechanism of ageing and development 94:123-134).
Apoptosis is needed to remove cells that represent a threat to the integrity of the organism. For example, it is the mechanism by which cytotoxic T lymphocytes (CTLs) kill virus-infected cells (Barber G N. 2001. Cell Death Differ. 8:113-126.). CTLs can induce apoptosis even in each other, so they can be eliminated after completion of their physiological function, preventing their becoming a liability for surrounding tissue (Duke R C. 1992. Semin Immunol. 4:407-412).
The anterior chamber of the eye and the testes are known as “immune privileged” organs, as it has been found antigens do not elicit an immune response in these sites. In these sites, the cells constitutively express high levels of Fas ligand (FasL), a cytokine that binds to a cell-surface receptor named Fas (also called CD95) and known as a potent death activator. FasL is toxic to T cells, and thus permits the prolonged, and sometimes permanent, survival of foreign tissue and tumor grafts (inhibited apoptosis) (Niederkorn J Y. 2002. Crit. Rev Immunol. 22:13-46; Takeuchi T. et al. 1999. J. Immunol. 162:518-522; Sugihara A, et al. 1997. Anticancer Res. 17:3861-3865). Thus, upregulation of apoptotic processes in specific cells, for example lymphocytes, can be useful in the prevention of graft rejection, potentially leading to reduction in the use of immunosuppressive drugs and improvement in the quality of the patient's life.
A variety of diseases have been associated with regulation of apoptosis. Among them are various neurodegenerative diseases, among them Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, retinitis pigmentosa, and epilepsy, all associated with selective apoptosis of the neurons. This neuronal death appears to be associated to increase susceptibility to apoptosis in these cells.
Mature blood cells are derived from haematopoietic precursors located in the bone marrow. Haematopoiesis, as well as maintenance of mature blood cells are regulated by a number of trophic factors (erythropoyetin, colony stimulating factors, cytokines). The balance between hematopoietic cell production and elimination is regulated by apoptosis. Loss of apoptosis regulation can be associate with a variety of blood disorders e.g. aplastic anemia, myelodyplastic syndrome, CD4+ T cells lymphocytopenia and G6PD deficiency.
In myocardial infarction and cerebrovascular accidents, ischaemic renal damage and polycystic kidney the cells surrounding the ischaemic zone are eliminated through apoptosis.
There is evidence showing that apoptosis is upregulated in a variety of cells e.g. neurons, myocytes, lymphocytes, hepatocytes, and that aging enhances apoptosis under physiological conditions that cause homeostasis dysfunction, such as oxidative stress, glycation, and DNA damage.
Apoptosis has been described in inflammatory cells (eosinophils, neutrophils, lymphocytes, macrophages, mast cells) that participate in the late and chronic stages of allergy (Sampson A P. 2000. Clin Exp Allergy. 30 Suppl 1:22-7; Haslett C. 1999. Am J Respir Crit. Care Med. 160:S5-11). For example, apoptotic death of the eosinophils is associated with bronchial asthma, allergic rhinitis and atopic dermatitis (Wooley K L et al. 1996. Am J Respir Crit. Care Med; 154: 237-243; Boyce J A. Allergy Asthma Proc. 18: 293-300). Lymphocytes apoptosis may induced by allergens, such as Olea europaea and Lolium perenneinduce (Guerra F et al. 1999. Hum Immunol; 60: 840-847).
Diseases associated with inhibition of apoptosis include those diseases in which an excessive accumulation of cells occurs (neoplastic diseases, autoimmune diseases). Where it was once believed that the excessive accumulation of cells in these diseases was due to an increased cell proliferation, it is now thought to be due to defective apoptosis.
In both solid and haematological tumors, the malignant cells show an abnormal response to apoptosis inducers (Watson A J M. 1995. Gut 37: 165-167; Burch W. et al. 1992. Trends Pharmacol Sci 13:245-251). In these diseases cycle-regulating genes such as p53, ras, c-myc and bcl-2 suffer mutations, inactivation or dysregulations associated to malignant degeneration (Merrit A J et al. 1994. Cancer Res 54:614-617; Iwadate Y et al. 1996. Int J Cancer 69:236-240; MüHauer L et al. 1996. Hepatology 23: 840-847; Newcomb E W. 1995. Leuk Lymphoma 17: 211-221). The expression of bcl-2 is considered to be a predictive factor for worse prognosis in prostate and colonic cancer and in neuroblastoma (Thompson C B. 1995. Science 267: 1456-1462). It has been shown that a number of antineoplastic therapies induce apoptosis in tumour cells (for reviews see: Sun S Y et al. 2004. J Natl Cancer Inst. 96:662-672; Schulze-Bergkamen H and Krammer P H. 2004. Semin Oncol. 31:90-119; Abend M. 2003. Int J Radiat Biol. 79:927-941).
Defects in the apoptosis may lead to autoimmune diseases such as lupus erythematosus (Carson D A. and Rebeiro J M. 1993. Lancet. 341: 1251-1254. Aringer M. et al. 1994. Arthritis Rheum. 37:1423-1430), rheumatoid arthritis (Liu H. and Pope R M. 2003. Curr Opin Pharmacol. 3:317-22.) and myasthenia gravis (Masunnaga A. et al. 1994. Immunol Lett. 39: 169-172.).
There are several ways by which the pathogens interfere with apoptosis. For example adenovirus and Epstein-Barr virus (associated with several lymphoid and epithelial malignancies) promote expression of Bcl-2 oncogene (Thompson C B. 1995. Science 267:1456-1462; Marshall W L. et al. 1999. J. Virol. 73:5181-5185), cowpox encode a protease inhibitor that inactivates caspases (Deveraux Q L, et al. 1999. J Clin Immunol. 19:388-98.); chlamydia interferes with mitochondrial cytochrome c release into the cytosol (Fan T. et al. 1998. J Exp Med. 187:487-496).
In chronic inflammatory, hyperproliferative skin diseases such as psoriasis, an abnormally low rate of apoptosis contributes to the development of epidermal hyperplasia. It was shown that keratinocytes respond to a variety of external and internal growth factors, including some proinflammatory cytokines which may suppress keratinocytes apoptosis, such as IL-15 (Ruckert R. et al. 2000. J. Immunol. 165:2240-2250).
Actin and Cell Motility
Actin is ubiquitous in nature, comprising the cytoskeleton and providing motility in all types of cells. The cellular actin cytoskeleton is organized in a variety of spatially and temporally controlled assemblies of actin filaments. Actin filaments are polymerized from monomeric G-actin in lamellipodia and filopodia at the cell periphery. These newly polymerized actin filaments are highly dynamic and are turned over rapidly (Wang, 1985). The actin filaments found in the remainder of the cells have their origin in the lamellipodium and in small membrane ruffles occurring throughout the lamella. Actin filaments are organized into various arrays such as stress fibers, lamellipodial networks, filopodial bundles, dorsal arcs, peripheral concave or convex bundles as well as geodesic arrays (Small, et al. Trends in Cell Biol 2002; 12:112-20). The organization of each of these assemblies is controlled and stabilized by specific sets of actin-associated proteins, conferring on them different functions. An asymmetric and polarized organization of the different actin arrays in cells is fundamental for cell migration, growth, division, differentiation, and defense (Hilpela et al, Mol Cell Biol 2003; 14:3242-53).
Cell motility depends on the cyclic dynamics of polymerization and depolymerization of the actin cytoskeleton. Cell motility involves protrusion of a cell front and subsequent retraction of the rear. Protrusion is based on the forward, cyclic growth, or polymerization of actin filaments in lamellipodia and filopodia. Retraction, on the other hand, is based on the interaction of preformed actin filaments with myosin-II in contractile bundles. Microscopic studies have shown a continuum of retrograde flow of actin behind lamellipodia, indicating that a proportion of filaments generated in the lamellipodium contribute to the network of actin that makes up the rest of the actin cytoskeleton. Thus, actin filaments are generated in the lamellipodia, shed their associate proteins as they become incorporated into the actin cytoskeleton, and then acquire other actin-associated proteins, particularly contractile proteins such as myosin-II, becoming contractile bundles in preparation for retraction of the cell protrusions.
Cell shape change and motility are involved in pathological events, such as cancer metastasis, inflammatory disease, neurodegenerative disease and the like. Cell motility associated proteins have been identified in the pathogenesis of a number of diseases, such as Wiskott-Aldrich Syndrome (WAS protein).
A large and growing number of proteins are known to regulate and modulate the state of the actin cytoskeleton, and some appear to have partly overlapping functions. These include actin and integrin binding proteins such as filamin, talin, Arp 2/3 complex, α-actinin, filament severing proteins and barbed end capping proteins (for review, see Brakebusch et al, EMBO Journal, 2003; 22:2324-33). In addition, there exist proteins of different upstream signaling pathways leading to changes in the actin cytoskeleton and cell morphology and behavior such as the small Ras-related GTPases, e.g., Rac, Rho, and Cdc42. In addition to these small GTPases, phosphoinositides and calcium are known to regulate actin dynamics and cell migration.
Presently, very few specific inhibitors of cell motility are available, even though a great potential exists for such drugs as a complement to existing therapies for inflammatory disease, cancer, neurodegenerative disease and the like. For example, cell shape change and motility are involved at two rate-limiting steps in cancer progression: angiogenesis (i.e., blood vessel recruitment) and metastasis (i.e., spreading of a tumor from one location in the body to other locations), in the extravasation of lymphocytes from vascular elements in inflammatory disease, and in the invasive progression of many cellular parasites into infected host tissue. In combination with cell growth inhibitors, treatment with specific cell motility inhibitors has the potential to provide a more efficacious treatment of diseases of cell motility and proliferation such as inflammatory disease, cancer, infections and the like, analogous to the multiple drug approach for treatment of HIV infection and AIDS.
A number of compounds that target actin directly are known to be effective in modulating cell motility. The best known compounds are the cytochalasins, which are cell-permeable destabilizers of actin filaments, and phalloidin, which is a cell-impermeable stabilizer of actin filaments (J. A. Cooper, J. Cell Biol, 105 (1987)). In addition, latrunculins are cell-permeable disrupters of actin filaments (I. Spector, Science, 219, 493 (1983)). Jasplakinolide is a cell-permeable stabilizer of actin filaments (M. R. Bubb et al., Chem., 269, 14869 (1994)). A few compounds that target proteins upstream of the actin cytoskeleton are known, such as the Rho-kinase inhibitor Y-27632 (M. Uehata et al., Nature, 389, 990 (1997), and myosin light chain kinase inhibitors, such as ML-g (M. Saitoh et al., Biochem. Biophys, Res. Commun., 140, 280 (1986)). Recently, a cyclic peptide dimer was discovered that inhibits the activity of N-WASP, a protein involved in Cdc42-mediated actin nucleation by the Arp2/3 complex (J. R. Peterson et al., Proc. Natl. Acad. Sci. USA, 98, 10624 (2001)). Nevertheless, there is a dearth of available compounds that affect actin dynamics and cell motility.
There is thus a widely recognized need and it would be highly advantageous to have a ribonuclease of the T2 family having actin binding activity, that has potential usefulness in the treatment and prevention of cell motility-associated disease such as inflammatory disease, cancer, neurodegenerative disease and infectious disease.